Dynamic Simulation for Optimising Pigging Frequency for Dewaxing

Abstract

The presence of paraffin in crude oil often leads to wax deposition in pipelines when temperatures reach below Wax appearance temperatures. It is possible to simulate the wax build up using thermodynamic and kinetic/deposition models for estimating wax deposition with varied degree of accuracy. The most popular remediation technique for wax removal is direct mechanical cleaning using pigs as these are generally cheaper than chemical inhibitors. The pigging frequency is mostly determined using thumb rules and operating experience. Correct pigging frequency is essential to avoid significant wax build-up in pipeline that may lead to plugging or stuck pig. This becomes more acute in subsea lines where insufficient well head pressures driving the pig may not scrap out the wax plug ahead of pig. This can increase to downtime and can lead to expensive pipeline repair. In Mumbai offshore pigging is most common method of wax removal in well fluid lines. In this paper a simulation model using dynamic simulator for predicting daily wax build-up and pigging shall be discussed. The model is based on the actual pipeline condition, fluid parameters and pigging data. Extensive lab studies on actual comingled well fluid samples from platform have been carried out for capturing the Fluid & Wax properties. The model has been used to run what if scenarios by varying the pigging frequency for the pipelines. The impact of ambient temperature or increase in gas (lift gas) in well fluid on pigging has been studied. Sensitivity studies have been carried out for various parameters like wax plug friction, bypass opening, change in well fluid parameters, etc.

The problems of formatoin of asphalt-resin-paraffin deposits. Prevention and utilization

“…Typically, deposits are removed without prior analysis of the reasons of their formation and accumulation in various units of industrial equipment [2]. The creation of a well-founded kinetic model of ARPD formation and growth will allow to develop ways of deposition minimization, and in the future it will allow to prevent their formation.…”

“…Oil characteristics of the content of potentially solid components are presented in Table 1. ARPD formation was studied using a method of «coldfinger test». A glass of oil (200 ml) was placed in a thermostatic bath, where with stirring at a speed of 120 Rev/min it was heated to the temperature of 30, 40 and 50 о C. The copper tube («coldfinger») with the surface of 744.45 mm 2 was cooled with running water to a temperature of 10 о C. Preliminarily weighed and degreased tube was placed in the center of the glass with oil so that it was at the height of 3 cm from the bottom. The tube was removed from the oil after 5 min and weighed.…”

“…Substitution of the values of the constants from Table 5 in the equation (2) for the exponential model as more accurate, allows to obtain the kinetic equation relating the rate of the deposits growth with temperature factor T*: To assess the influence of the layer thickness of the deposit formation on the deposition rate, the dynamics of single (20 min) formation of the deposit were compared with cumulative (with intermediate unloading every 5 minutes) formation ( Fig. 2).…”

“…It is natural that the growth of ARPD layer on the surface of the tube prevents heat transfer between the cooled surface and the tube, distorting the true temperature gradient set in the experiment. The results of the model thermal calculations performed in accordance with the methods [2] are given in table 6. Table 6 The temperature regime of ARPD deposition, depending on the hydrodynamics and the layer thickness of the deposit at the temperature of the tube 10 о C Oil Data shown in Table 6 indicate that in the turbulent regime the increase in the thickness of the deposit on the surface of the tube to 1 mm cuts the actual temperature difference by half, and when the layer thickness is 2 mm -by three.…”

“…Dynamics of ARPD accumulation from oil at the temperature difference between the oil and the surface of the tube, △T, o С: 1 -40; 2 -30; 3 -20. Points correspond to experimental data, dashed lines are the calculated curves according to the equations (1),(2) …”